How do the Mackerel and the mackerel-agent communicate? mackerel -agent communicates with the Mackerel via its HTTP REST API. No direct communication is done from the Mackerel side to the host that contains mackerel-agent. Encrypted with SSL/TLS correspondence. Related articles Is connectivity monitoring available for managed services like Amazon RDS? Is it possible to control which port the mackerel-agent uses? What is Mackerel's IP address and port number?